1. Field of the Invention
The present invention relates to an electronic weighing apparatus. More specifically, the present invention relates to an improved electronic weighing apparatus wherein an analog voltage signal obtainable from a load transducer, such as strain gauge type load cell, or from a differential transformer or the like, is converted into a digital signal representing the weight of an article being weighted.
2. Description of the Prior Art
In general, a conventional electronic weighing apparatus comprises a load detector for providing an output voltage proportional to the weight of an article being weighted. The detector may be a load cell and a preamplifier is used for amplifying the output voltage from the load cell by a predetermined amplification factor, an analog-to-digital converter is used for converting the analog output voltage from the preamplifier into a digital signal, and a digital display indicates the digital signal from the analog-to-digital converter in a digital manner. Another type of weighing apparatus is also known, wherein a mechanical displacement is caused in proportion to the weight of an article being weighed, a pulse output is generated by a pulse generator responsive to the mechanical displacement, and the number of pulse ouput pulses is counted, to provide a digital signal associated with the weight of the article. However, such a weighing apparatus comprising a mechanical displacement means, a pulse generator and a pulse counter is subject to mechanical wear and external noise disturbance, which could cause an error and malfunction. By contrast, an electronic weighting apparatus employing a load cell and an analog-to-digital converter is free from such shortcomings. Nevertheless, an electronic weighing apparatus employing a load cell and an analog-to-digital converter suffers from other inherent problems to be discussed below.
FIG. 1 is a graph showing an ideal analog input versus digital output transfer function characteristic curve of an electronic weighing apparatus. The stepwise function shows the relation of the digital output Di on the ordinate and the analog input Wi along the abscissa. A straight line a running from the zero point toward a full scale point shows an ideal line of an infinite resolution analog-to-digital converter. Ideally, it is desired that the straight line "a" crosses the step or stage portions of the stepwise curve at the center of the respective step or stage portions. Nevertheless, with a conventional electronic weighing apparatus, it is extremely difficult or rather impossible to position the ideal straight line "a" with respect to the stepwise characteristic curve in such an ideal manner as described above. Thus, in general, it is usual that an error is caused between the actual analog input value Wi and the digital output value Di. Such an error is often referred to as a differential non-linearity error.
More specifically an analog-to-digital converter has regular stepwise transfer function with respect to the analog voltage input Wi and the digital value output Di. One digital value represented by a single step or stage in the curve corresponds to a predetermined range of the analog value. Most ideally, the amount of diversion by the stepwise curve from the ideal straight line "a" in the upward direction and the amount of diversion by the stepwise curve from the straight line "a" in the downward direction should be the same. In other words, the analog input value representative of the weight having the digital value output Di preferably satisfies the following equation whereby the analog input value Wi corresponds to the intersecting point of the ideal straight line "a" and the digital numerical value output Di; EQU Wi= .+-. 1/2 LSB
where LSB is an abbreviation of the least significant bit and denotes an error indicating unit for the analog-to-digital conversion, which is defined as a variation amount of the analog input necessary for the variation of one digital numerical value output. For example, assuming that the analog load input of 0 to 999 tons is converted into a digital numerical value of 0 to 999, one LSB is one ton.
However, the FIG. 1 graph shows merely an ideal analog input versus digital output characteristic. In general, most analog-to-digital converters now commercially available and employed in electronic weighing apparatuses have not been rated relative to such an ideal characteristic as shown in FIG. 1. The reason is that the analog-to-digital converters now commercially available have a inherent accuracy which is subject to an error affected by the external conditions, such as the ambient temperature and the like. Usually, even the best analog-to-digital converters comprises an error of .+-. one LSB, as shown in FIG. 2, which shows a possible diversion of the stepwise analog input versus the digital output characteristic curve, from the ideal straight line "a". For instance, the vicinity of the zero point is considered as an important example in the solid line characteristic curve "b", if and when the analog value falls in the range of zero to plus one, the corresponding digital value of zero is obtained, whereas if and when the analog value falls in the range of zero to minus one, the digital value is minus one. Accordingly, if the analog value moves a little from zero in the plus direction, the digital value is zero, whereas if the analog value moves a little from zero in the minus direction, the digital value suddenly becomes minus one. Thus, it is clear that the analog value is converted into an inaccurate digital value, in case of the solid line characteristic curve.
In actuality, the direct opposite situation as shown by the dotted line b' in FIG. 2 could occur by way of another extremity of diversion of the characteristic curve. In other words, diversion of the stepwise analog input versus digital output characteristc with respect to the ideal straight line "a" could occur depending on the mechanical and electrical inherent characteristic of the various components in the electronic weighing apparatus. The solid line stepwise characteristic curve "b" shown in FIG. 2 can be considered as if the fraction in the least significant digit value or the value of the digit that is less significant than the decimal point of the digital output has been processed to be discarded. On the other hand, the dotted line stepwise characteristic curve b' can be considered as if the fraction in the least significant digit value or the value of the digit that is less significant than the decimal point has been processed to be carried over to a more significant digit position. Most idealy, every electronic weighing apparatus should be able to process the fraction in the least significant digit value or the vaue of the digit that is less significant than the decimal point of the digital output such that the value smaller than 0.5 in the least significant digit of the digital output is discarded while the value exceeding 0.5 in the least significant digit of the digital output is carried over to the more significant digit. Nevertheless, in a conventional electronic weighing apparatus, such processing of the fraction in the least significant digit of the digital output has been effected solely depending on the mechanical and electrical inherent errors of the components in the apparatus. In other words, in a conventional electronic weighing apparatus, the stepwise analog input versus digital output characteristic curve is positioned with respect to the ideal straight line a in a diversified manner within the diversion range between the dotted and solid lines b' and "b" in the FIG. 2 analog input versus digital output characteristic, so that such diversion is determined by the electrical and mechanical inherent factors of the respective apparatus. In converting an analog input into a digital signal, it has been common to try to minimize the so called differential non-linearity error. To that end, it is necessary that the respective step or stage portions in the stepwise characteristic curve are crossed by the ideal straight line "a" substantially at the center of the respective step or stage portions. Since such a requirement has not been satisfied by the conventional electronic weighing apparatus and the so called analog input versus digital output characteristic has been diversified with respect to the ideal straight line "a", as discussed with reference to FIG. 2, it could happen that even an extremely minor change of the weight caused, for example, by a draft, slight shock or the like could cause the display to indicate the minimum digital value of unity and thus cause the viewer to a notice flickering in the display.
As described above, it is extremely difficult to achieve a uniform analog input versus digital output characteristic by means of analog-to-digital converters, and thus in an electronic weighing apparatus. Therefore, there is a limit to the accuracy of prior art electronic weighing apparatus. Nevertheless, legislation, for example, could provide for a strict requirement of accuracy to be satisfied by an electronic weighing apparatus used for general commercial transactions. Since the conventional electronic weighing apparatus has not been designed to meet such a requirement, a diversified analog input versus digital output characteristic of the apparatus could make the customer to feel distrustful of such electronic weighing apparatus used in general commercial transactions.
Apart from the foregoing discussion, in general, an analog-to-digital converter inherently comprises a nonlinearity error. Hence, it is desired that such a nonlinearity error should be minimized, if possible at all.
The above described various problems could be solved by implementing an improved electronic weighing apparatus that is capable of achieving an ideal analog input versus digital output transfer function characteristic curve as shown in FIG. 1. Such a requirement could be achieved by providing a direct current bias to a preamplifier for amplifying an analog output from a load cell in a conventional electronic weighing apparatus, thereby to adjust the level of the analog signal, as desired. However, it can be readily appreciated that such adjustment would be extremely tiresome and time consuming and nevertheless the characteristic, as thus adjusted, would still be subject to external disturbances such as variation of the ambient temperature or the like.